The Role of Extracellular Ca2+ and Formation and Duration of Pores on the Oolemma in the Electrical Activation of Mouse Oocytes.

Abstract

Mouse oocytes were pulsed in 300 mM mannitol solutions containing (MS+) or not containing (MS-) Ca2+ and stored at different room temperatures in M2 with (M2+) or without (M2-) Ca2+ for different periods of time before culture in Whitten medium. The role of extracellular Ca2+, the formation and duration of pores on the oolemma, and the effect of low temperature during oocyte activation were studied by measuring the rate of oocyte activation. The results were as follows: 1. At 20 C, when oocytes were pulsed in MS+ and transferred to and stored in M2+ for 10 min, the activation rate was 61.9%, significantly (P<0.01) higher than that obtained when oocytes were pulsed in MS- and stored in M2- for 10 min (1.8%), indicating that extracellular Ca2+ was absolutely necessary for oocyte activation. 2. When oocytes were pulsed in MS- and stored in M2- for 10 min, 5 min and 30 sec before transfer to M2+, the activation rates were very low, being 1.8, 6.7 and 3.8%, respectively, significantly (P<0.05) lower than that (16.4%) when oocytes were transferred to M2+ immediately (within 15 sec) after pulsing in MS-, suggesting that pores were formed on the oolemma during pulsing and they began to close within 30 sec after stimulation. 3. When oocytes were pulsed in MS+ at 4 C and stored in M2+ for 5 min at 4 C, the activation rate was only 22.5%, significantly (P<0.05) lower than that (61.9%) of the oocytes treated under the same conditions but at 20 C. When oocytes pulsed in MS+ at 4 C were stored in M2- for 5 min and 10 min at the same temperature, the activation rates were 25.8 and 10.8%, respectively, and the difference between them was significant (P<0.05). Although there might be other reasons for the low activation rate in 4 C experiments, these two results might indicate that the reduction in fluidity of the oolemma at low temperature made the pore formation more difficult, but the pores persist longer than at normal temperature.